It sounds like you might have fungus gnats which are usually caused by soil that is kept too moist. Overwatering might also be the cause of yellowing and dropping leaves. The lifecycle of gnats is only a few days so if you allow the soil to dry out more between waterings, they should quickly disappear.
Using pebbles at the bottom of pots actually impairs drainage. If the pots don't already have drainage holes, repot the plants
into ones that do, the smallest size that will accommodate the root balls.
Watering according to a schedule can cause overwatering, too. Water requirements are change with variables such as amount of light, heat, and humidity. Shorter days of winter means your plants
will need less water. A simple, yet accurate, method of determining if your Schefflera arboricolas need watering is to poke your finger into the soil. When the soil is dry about 1/4 deep, it is time to water.